1. Field of the Invention
The present invention relates to a surface conduction electron emitting device, an electron source provided with a surface conduction electron emitting device, and an image forming apparatus provided with an electron source. The present invention also relates to a method of producing such devices.
2. Related Background Art
Electron emitting devices are roughly classified into two types: a thermionic emission type and a cold-cathode emission type. Electron emitting devices of the cold-cathode emission type are further classified into several types. They include a field emission type (hereafter referred to as an FE type), a metal/insulator/metal type (hereafter referred to as an MIM type), and a surface conduction electron-emitting type. Examples of FE types are disclosed for example in "Field Emission" (W. P. Dyke and W. W. Dolan, Advance in Electron Physics, 8, 89 (1956)) and "Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones" (C. A. Spindt, J. Appl. Phys., 47, 5248 (1976)).
An example of an MIM type has been reported by C. A. Mead in his paper entitled "Operation of Tunnel-Emission Devices", J. Apply. Phys., 32, 646 (1961).
An example of a surface conduction electron emitting device has been reported by M. I. Elinson (Radio Eng. Electron Phys., 10, 1290 (1965)).
Surface conduction electron emitting devices use a phenomenon that electron emission occurs when a current is passed through a thin film with a small area formed on a substrate so that the current flows in a direction parallel to the film surface. Various types of surface conduction electron emitting devices are known. They include a device using a thin SnO.sub.2 film proposed by Elinson et al. as aforementioned, a device using a thin Au film (G. Dittmer, Thin Solid Films, 9, 317 (1972)), a device using a thin In.sub.2 O.sub.3 /SnO.sub.2 film (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519 (1975)), and a device using a thin carbon film (Araki et al., Vacuum, 26 (1), 22 (1983)).
A typical surface conduction electron emitting device is schematically shown in FIGS. 2A and 2B wherein FIG. 2A is a plan view and FIG. 2B is a cross-sectional view. As shown in FIGS. 2A and 2B, the device includes a substrate 1, device electrodes 2 and 3, a conductive thin film 4, and an electron emitting region 5. The electron emitting region 5 is formed by conducting a current through the conductive thin film 4 after forming the device electrodes 2 and 3 and the conductive thin film 4 on the substrate 1. This process is known as an energization forming process. In the energization forming process, a voltage is applied between the device electrodes 2 and 3 so that a current flows through the conductive thin film thereby introducing local breakage, deformation, or qualitative change in the conductive thin film and thus forming an electron emitting region 5 having a high electric resistance. In the electron emitting region 5, a fissure or fissures are formed in a part of the conductive thin film and electrons are emitted from the fissure(s) or regions near the fissure(s) when a voltage is applied between the device electrodes so that a current is passed through the conductive thin film.
The methods of forming the device electrodes and the conductive thin film, the energization forming process of forming the electron emitting region, and other processes are disclosed for example in Japanese Patent Application Laid-Open No. 7-235255.
The electron emitting device of the surface conduction type has a simple structure and thus can be easily produced. Therefore, it is possible to dispose a great number of similar devices over a large area. Because of these advantages, a lot of research and development activities are being made to apply the surface conduction electron emitting device to various apparatuses and systems such as a charged-beam source, an image display device, etc. For example, an electron source having a large number of surface conduction electron emitting devices has been reported in which a plurality of electron emitting devices are disposed along a line called a device row and a plurality of similar device rows are disposed wherein in each device row, one electrode of each electron emitting device is connected in common to an interconnection, while the other electrode of each electron emitting device is connected in common to another interconnection (for example refer to Japanese Patent Application Laid-Open No. 64-031332, Japanese Patent Application Laid-Open No. 1-283749, Japanese Patent Application Laid-Open No. 2-257552).
In recent years, a flat panel type image forming apparatus using a liquid crystal (LCD) has come to be widely used as an image display device instead of a cathode-ray tube (CRT). However, LCDs are not a device of the emission type and thus have the disadvantage that a back light is required. Thus, there is a need for a display device of the emission type. One known technique to realize a display device of the emission type is to employ an electron source provided with an array of a great number of surface conduction electron emitting devices to excite a fluorescent screen thereby emitting visible light. This technique is disclosed for example in U.S. Pat. No. 5,066,883.
When an electron emitting device is used in practical applications, it is required that good electron emission characteristics be maintained for a long time without instability.
In surface conduction electron emitting devices, two important characteristics are the magnitude of electron emission current (denoted by Ie) and the electron emission efficiency (.eta.).
The electron emission efficiency refers to the ratio of the emission current Ie to the current (device current If) flowing between the device electrodes, that is, .eta.=Ie/If.
To use a surface conduction electron emitting device in a practical application, it is required that the magnitude of the emission current and the electron emission efficiency be maintained at constant values for a long time without instability. Besides, it is desirable that the device can provide a large emission current and a high electron emission efficiency.
For example, when a surface conduction electron emitting device is employed in an image forming apparatus, the emission current Ie should be great enough to achieve a sufficiently bright image. If the electron emission efficiency .eta. is high enough, then it is possible to achieve a bright image with low electric power consumption. This results in a reduction in the load of a driving circuit, which allows a reduction in the total cost.
The above requirements are not met satisfactorily in the conventional surface conduction electron emitting devices, and it is still required to increase the emission current Ie and the electron emission efficiency .eta. and it is also required to improve the stability of the electron emission characteristics.